Deletion of both syntaxin-2 and syntaxin-4 (Stx2/4) does not inhibit platelet secretion. (A) Platelet extracts (5.0 × 10⁷ platelets/lane) were prepared from wild-type (Wt) and syntaxin-2 and syntaxin-4 double-knockout (Stx2/4 DKO) mice and the indicated proteins were detected by Western blotting. [³H]-serotonin–labeled platelets from Stx2/4 DKO (gray circle symbols) and Wt (black square symbols) were prepared as described in “Methods.” Release of [³H]-serotonin from dense granules, PF4 from α-granules, and β-hexosaminidase from lysosomes was measured as a thrombin-response curve (1-minute stimulation; B) or as a time course (0.05 U/mL of thrombin; C) and calculated as described in the legend to Figure 1.